The countries of the African Great Lakes Region have an agriculture-based economy. The East African highland banana is a key staple crop for the local population, providing both food and cash. At regional scale, on-farm banana yields are low (5-30 t.ha-1.year-1), and yield declines have been repeatedly reported. Farmers and scientists often stated that poor and declining soil fertility is one of the dominant factors hampering the production. Systems are under increasing pressure due to
(i) decreasing farm sizes due to the growing population, and (ii) the non-use of fertilizer combined with encouragements to have farmers sell their produce to the (local) market. In these constrained systems, farmers’ management of organic matter inputs is suggested to be a key factor driving soil fertility evolution.
By combining field observations and measurements, classic laboratory analyses on soil and plant samples, and X-Ray Diffractometry on fine earth and textural fractions (sand, silt and clay), we attempted to evaluate the contribution of geology and human activities to the soil fertility in four banana-producing regions around the African Great Lakes (Ntungamo (SW Uganda), Butare, Kibungo and Ruhengeri (all in Rwanda)).
The soils at Ruhengeri are Andosols developed on volcanic rocks. Weatherable primary minerals still remain in those soils, explaining their high total reserves in bases (TRB, mean=382meq/100gr (N=40)). These soils contrast with the Acrisols or Nitisols of Ntungamo, Butare or Kibungo. There, TRB is relatively low (24-128meq/100gr, mean=64meq/100gr (N=168)) as soils are old and weathered. Differences in parent rocks in the latter three regions (resp. micaschist, granite and pelitic rocks) stand out by an opposition between the soils of Ntungamo and Butare, on one hand, and those of Kibungo, on the other hand. The former are rich in silica (resp. on average 77 (N=30) and 70%SiO2 (N=102)) and total iron represents only 3-5%wt in the fine earth. The soils in Kibungo have a lower SiO2 content (mean=39%, N=38) and contain more crystalline iron oxides (mean=13%Fe2O3 (N=38); (Feo/FeDCB)mean= 4% and (FeDCB/Fetot)mean= 77% (N=11)). Stabilization of organic matter by iron oxides could be hypothesized to explain the higher carbon contents in the Kibungo soils and hence their higher global fertility (higher pH, CEC, exchangeable bases content). The contribution of minerals and organic matter to soil nutrient content has also been assessed by the comparison between the nutrient contents in surface horizons and those at depth. For the three regions, total Ca, Mg and K contents (resp. 9.4-27.8, 9.3-16.7, 11.6-37.8 meq/100gr) are usually higher in surface horizons than at depth (>100cm, resp. 1.5-6.2, 7.5-8.8, 5.2-46.6 meq/100gr) where organic matter content is significantly reduced. Organic matter contributes to soil fertility as a nutrient source but also through its influence on the chemical and physical properties of soils. Hence, total carbon content (%Ctot) and cation exchange capacity (CEC) are intimately related (CEC = 4.26%Ctot + 0.06%clay + 0.94 (R²=0.89; N=84)). For surface horizons, %Ctot explains between 55 and 89% of CEC.
In the old and weathered soils of the Great Lakes Region, organic matter is a major contributor to soil nutrient contents, both as a source of nutrients as through its influence on nutrient retention. In the prevailing low-input systems, the management of organic resources is thus a key issue in maintaining the soil fertility and in insuring the sustainability of the system.